x86, cpu: Only CPU features determine NX capabilities

Fix the NX feature boot warning when NX is missing to correctly
reflect that BIOSes cannot disable NX now.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
LKML-Reference: <1289414154-7829-5-git-send-email-kees.cook@canonical.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>